The present invention relates to an article with a photocatalytic film, such as an automotive or architectural window pane. The article is provided with anti-fogging function, stain resistance, hydrophilicity and good visibility in rainy weather.
Japanese Patent Laid-open Publication JP-A-9-328336 discloses a composition for forming a photocatalytic film. This composition comprises (1) TiO2 grains having an average grain diameter of less than 100 nm, (2) Zr-containing compound, and (3) Si-containing compound. The composition is such that the weight ratio of the Zr-containing compound to the TiO2 grains is from 0.02 to 0.5 in terms of oxide and that the weight ratio of the Si-containing compound to the TiO2 grains is from 0.2 to 2.5 in terms of oxide.
JP-A-10-216528 discloses a photocatalytic article comprising a substrate and a catalytic film formed thereon. This catalytic film is a combination of photocatalytic grains and a binder containing 60-90 wt % of silica and 10-40 wt % of zirconia.
JP-A-9-227159 discloses a vehicular front or rear window glass having a substantially transparent layer bonded to a substrate. This layer contains a photocatalytic semiconductor material.
JP-A-10-156999 discloses a photocatalytic layer made of a mixture of silica and titania. JP-A-9-271731 discloses a photocatalytic member having a top surface comprising titanium oxide and silicon oxide as major components. JP-A-9-56788 discloses an article for bathroom, having a photocatalytic layer made of a mixture of silica and titania.
JP-A-11-35342 discloses a multi-functional glass having a soda-lime glass substrate, a first layer formed on the substrate, and a second layer formed on the first layer. The first layer is a SiO2 film containing at least one member selected from Ti and metal elements having electronegativities close to that of Ti. The second layer is a TiO2 film or a SiO2 film in which TiO2 fine particles are dispersed.
It is an object of the present invention to provide an article with a photocatalytic film, which is improved in photocatalytic activity, abrasion resistance and chemical resistance.
It is another object of the present invention to provide a process for producing such article.
According to the present invention, there is provided an article comprising a substrate; and a photocatalytic film covering said substrate. This film comprises (1) a film-forming component comprising ZrO2 and SiO2 and (2) TiO2 crystals dispersed in said film-forming component, said ZrO2, said SiO2 and said TiO2 crystals respectively being in amounts of 25-60 wt %, 15-50 wt % and 25-45 wt %, based on a weight of said film.
According to the present invention, there is provided a first process for producing the article. The first process comprises (a) providing a coating liquid comprising a first raw material of said ZrO2, a second raw material of said SiO2, and a third raw material of said TiO2 crystals; (b) applying said coating liquid to a surface of said substrate, thereby forming thereon a precursory film; and (c) subjecting said precursory film to a heat treatment at a temperature of 300-700xc2x0 C., thereby turning said precursory film into said photocatalytic film.
According to the present invention, there is provided a second process for producing the article. The second process comprises (a) providing a coating liquid comprising a first raw material of said ZrO2, a second raw material of said SiO2, and a third raw material of said TiO2 crystals; (b) applying said coating liquid to a surface of said substrate, thereby forming thereon a precursory film; and (c) subjecting said precursory film to a first baking at a temperature of 560-700xc2x0 C., thereby turning said precursory film into said photocatalytic film.
We unexpectedly found that an article according to the invention is improved in photocatalytic activity (e.g., stain resistance and hydrophilicity), abrasion resistance and chemical resistance (e.g., acid resistance and alkali resistance) by specifying respective amounts of ZrO2, SiO2 and TiO2 crystals as above, particularly by increasing the amount of ZrO2 relative to that of TiO2 crystals as above. For example, the obtained article becomes satisfactory in an alkali resistance test for Category A of Japanese Industrial Standard (JIS) R 3221 and thus can practically be used as an exterior window pane such as automotive or architectural window pane.
As stated above, the respective amounts of ZrO2, SiO2 and TiO2 crystals are specified. Particularly, TiO2 crystal fine grains, for example, having an average grain size of not greater than 30 nm, are preferably used as at least part of a raw material of TiO2 crystals of the photocatalytic film. Due to these features, the photocatalytic film does not have cracks even if the baking of the precursory film is conducted at a high temperature at which a glass plate (substrate) can be bent into a curved glass plate. Generally speaking, such high temperature tends to make a photocatalytic film inferior in photocatalytic activity. According to the invention, however, the photocatalytic film does not become inferior in photocatalytic activity by such high temperature, due to the above features. Therefore, it is not necessary to provide an alkali barrier layer between the glass plate and the photocatalytic film for preventing the movement of alkali components of the glass plate into the photocatalytic film.
The total amount of ZrO2, SiO2 and the TiO2 crystals of the photocatalytic film is preferably at least 90 wt %, based on the weight of the film, in order to have photocatalytic activity and durability such as abrasion resistance, acid resistance and alkali resistance. Besides these essential components, the film may optionally comprise not greater than 10 wt % of at least one component such as amorphous TiO2, colloidal silica, other oxides (e.g., Al2O3, B2O3 and SnO2), ions (Cr ion and V ion) and carbon. The TiO2 crystals of the film may have an average grain size of not greater than 30 nm or not greater than 20 nm.
In case that the article is used as an automotive window pane, it is preferable to dispose the photocatalytic or hydrophilic film on an outer side of automobile.
In case that a glass plate is used as the substrate in the second process, the glass plate may be bent into a curved glass plate after the first baking or when the precursory film and the glass substrate are subjected to the first baking.
In order to improve the film in transparency and durability, the third raw material of the first process is preferably TiO2 grains having an average grain size of not greater than 20 nm, and that of the second process is preferably those grains having an average grain size of not greater than 30 nm.
The first raw material of the first and second processes is preferably zirconium chloride or zirconium nitrate in order to improve the photocatalytic film in durability, particularly abrasion resistance, acid resistance and alkali resistance. Examples of the zirconium chloride include zirconium chloride, zirconium oxychloride octahydrate, and a chlorine-containing zirconium alkoxide represented by the general formula Zr(OCmH2m+1)xCly where m, x and y are integers and x+y=4. Examples of the zirconium nitrate include zirconium oxynitrate dihydrate. The ZrO2 content of the film is not lower than 25 wt % in order to have the film provided with alkali resistance and photocatalytic activity. If it is higher than 60 wt %, the film becomes inferior in abrasion resistance and/or photocatalytic activity. The ZrO2 content of the film is preferably from 25 to 45 wt %. As ZrO2 is added to the film, the film is improved in alkali resistance. Furthermore, the film is remarkably improved in photocatalytic activity if the ZrO2 content of the film is not lower than 25 wt %.
The second raw material of the first and second processes is not particularly limited, so long as it can produce SiO2 after the heat treatment or the first baking. Examples of the second raw material include alkoxysilanes such as tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane and methyltriethoxysilane, hydrolysates of these, polycondensates of these, those stabilized with stabilizers such as acetylacetone, and commercial products for producing SiO2 such as COLCOAT P of COLCOAT Co., MSH2 of Mitsubishi Chemical Corp. and CSG-DI-0600 of Chisso Co. The SiO2 content of the film is not lower than 15 wt % in order to have the film provided with abrasion resistance. If it is higher than 50 wt %, the film becomes inferior in photocatalytic activity and/or alkali resistance. The SiO2 content of the film is preferably from 25 to 45 wt %.
The third raw material of the first and second processes is not particularly limited, so long as it can produce TiO2 crystals after the heat treatment or the first baking. It is possible to use commercial powdery TiO2 fine grains for photocatalyst, such as ST-01 and ST-21 of Ishihara Techno Co., SSP-25 and SSP-20 of SAKAI CHEMICAL INDUSTRY CO., LTD., PC-101 of Titan Kogyo K.K., SUPER TITANIA F-6 and SUPER TITANIA F-5 of Showa Titanium Co., and DN-22A of FURUKAWA CO., LTD. It is further possible to use liquid-type commercial products of the third raw material, such as STS-01 and STS-02 of Ishihara Techno Co. and A-6 and M-6 of TAKI CHEMICAL CO., LTD. It is still further possible to use a mixture of TiO2 fine grains for photocatalyst and a raw material of silica, such as ST-K01 and ST-K03 (trade names) of Ishihara Techno Co. The TiO2 crystal content of the film is not lower than 25 wt % in order to have a photocatalytic capability to achieve hydrophilicity and stain resistance. It is not greater than 45 wt % from the viewpoint of alkali resistance and abrasion resistance. Powdery TiO2 fine grains can easily be dispersed in a coating liquid by a common mixing operation for dispersing a powder in a liquid, for example, using a ball mill. Upon this, it is optional to disperse the first raw material of ZrO2 and the second raw material of SiO2 in the coating liquid.
The manner of applying the coating liquid to the surface of a substrate is not particularly limited. It can be conducted by a common method such as dip coating, spin coating, roller coating, spraying, screen printing or the like.
In the first process, the precursory film is subjected to a heat treatment at a temperature of 300-700xc2x0 C., thereby turning the precursory film into the photocatalytic film. Prior to this heat treatment, it is optional to conduct a drying at a temperature not higher than 200xc2x0 C. or a preliminary baking at a temperature lower than that of the heat treatment. It is preferable to regulate the temperature of the heat treatment in a manner that a period of time, during which the heat treatment temperature lower than a maximum temperature of the heat treatment by 100xc2x0 C. or less, is at least 1 minute, preferably at least 3 minutes. This maximum temperature can be arbitrarily set within a range of 300-700xc2x0 C. The upper limit of the heat treatment time is not particularly limited. For example, a heat treatment time of 24 hr does not cause particular problems. The above-mentioned period of time is preferably about 2 hr from the viewpoint of productivity.
In the second process, the precursory film is subjected to a first baking at a temperature of 560-700xc2x0 C., preferably 600-700xc2x0 C., thereby turning the precursory film into the photocatalytic film. During the first baking, the substrate may be bent if it is necessary to produce a curved substrate. The film is provided with durability by the first baking. It is optional to conduct a second baking (preliminary baking) at a temperature of 300-620xc2x0 C., preferably 450-620xc2x0 C., prior to the first baking (main baking). A film becomes more compact and improved particularly in abrasion resistance by conducting these two bakings. In case that an automotive curved window pane is produced as the article of the invention, it is preferable to regulate the temperature of the first baking in a manner that a period of time, during which the first baking temperature lower than a maximum temperature of the first baking by 100xc2x0 C. or less, is at least 1 minute, preferably at least 2 minutes. This maximum temperature can be arbitrarily set within a range of 560-700xc2x0 C. The upper limit of the first baking time is not particularly limited. For example, the above-mentioned period of time is preferably about 3 minutes in case that a glass substrate is tempered by the first baking and preferably about 10 minutes in case that a glass substrate is bent during the first baking for producing a laminated glass. The TiO2 crystals in the form of fine grains are dispersed in the film-forming component of the film, and the TiO2 crystal content of the film is limited to a range of 25-45 wt %. With this, the film does not have cracks even if a glass substrate is bent during the first baking.
The film thickness is preferably from 30 to 500 nm in order to have photocatalytic activity and a high durability. It is more preferably from 50 to 300 nm in order to have a good transparency and a still high durability by the formation of a photocatalytic film one time. The material of the substrate is not particularly limited, as long as it does not deteriorate by heating. It may be selected from glass, ceramics and metals (e.g., aluminum and stainless steel). In particular, glass is preferable because of its transparency and heat resistance.
The article according to the invention is improved in durability, hydrophilicity, anti-fogging and stain resistance by the provision of the photocatalytic film. Thus, it can be used in various severe environments for building, vehicular, aircraft and industrial window panes, exterior tiles, exterior panels, showcases, and various mirrors such as vehicular door mirror and bathroom mirror. In particular, the article is preferably used for vehicular front, rear and side window panes. The article is improved in visibility under a rainy weather by the provision of hydrophilicity.
As mentioned above, the substrate can be a glass plate, for example, of a soda-lime-silicate glass. The glass plate is not limited to particular types. For examples, it may be a clear or colored glass plate having a color of blue, gray, bronze, green or the like, a glass reinforced with net or the like, a curved, semi-tempered or tempered glass, a double layer glass (sealed double-glazed unit) optionally having a metal film such as Low-E film, a double layer glass having a gel between two glass plates, a laminated glass, a glass plate pierced or formed on its uncoated side with a metal, oxide or resin film by deposition, sputtering, printing or the like, or a glass plate having an uncoated side subjected to etching, sand blasting or the like.
When the TiO2 crystals of the photocatalytic film are irradiated with ultraviolet rays contained in the sunlight or a fluorescent light, organic contaminants on the film are oxidatively decomposed by the photocatalytic activity of the TiO2 crystals. With this, the surface of the film becomes clean, and the surface of the TiO2 crystals becomes hydrophilic. A film not according to the invention made of only TiO2 crystals also becomes hydrophilic by its irradiation with ultraviolet rays. This film, however, becomes hydrophobic in a relatively short time due to the original hydrophobicity of TiO2, if the irradiation is not repeated. In contrast, according to the invention, SiO2 is added to the film. With this, the film is improved in hydrophilicity. Furthermore, a larger amount of water is adsorbed to the film, thereby improving the photocatalytic capability of the film. Still furthermore, the film is improved in durability such as abrasion resistance. According to the invention, ZrO2 is added to the film. With this, the film is further improved in durability, particularly abrasion resistance and alkali resistance, and in photocatalytic capability. Thus, according to the invention, the photocatalytic film is very much improved in photocatalytic capability and durability, particularly abrasion resistance and alkali resistance. The surface of the photocatalytic film is kept hydrophilic. The film is capable of decomposing organic contaminants (e.g. organic components of exhaust gas and dust) by the photocatalytic activity. Furthermore, stain on the film runs down easily by rain water, since the film surface is kept hydrophilic. Furthermore, even if a glass plate used as the substrate is bent during the baking, the film does not have cracks and have a high quality and a high durability.
The following nonlimitative examples are illustrative of the present invention.